In order for a rolling element bearing to operate in a reliable way it must be lubricated adequately. Lubricant prevents metal to metal contact within the bearing and protects surfaces within the bearing against corrosion. It is therefore important to select the proper lubricant and lubrication method for each individual bearing application, as well as a correct maintenance program.
In some instances, particularly concerning large machinery that is not easily accessible, such as wind turbines, it is desirable to provide a method of monitoring the lubrication condition of a bearing without affecting normal running of the bearing.
A method of monitoring the lubrication condition of a rolling element bearing by means of determining a lubrication parameter indicative of the lubrication condition is described in WO2010/085971.
In a first step, high-frequency structure-borne acoustic emissions are measured. The measured acoustic emissions are generated as a result of asperity contact between rolling surfaces of the bearing, and provide a measured signal. In a second step, emitted acoustic energy is extracted from the measured signal. In a third step, the lubrication parameter is determined from the emitted acoustic energy, on the basis of a power-law relationship between acoustic energy and the lubrication parameter. The lubrication parameter used to indicate lubrication condition is either specific lubrication film thickness (Lambda) or lubricant viscosity ratio (Kappa). The value of Kappa is the ratio of a lubricant's actual viscosity to a minimum required viscosity that the lubricant must possess in order to form an adequate lubricant film. The required viscosity is dependent on the size and speed of rotation of the bearing whilst actual viscosity is dependent on the lubricant and temperature.
In bearing life calculations the actual viscosity in the viscosity ratio (Kappa) is presumed from the viscosity grade of the lubricant, the lubricant operating temperature and bearing speed.
A bearing life model is a statistical model which says that under a certain set of operating parameters a certain percentage (for example 90%) of bearings of the same type and lubricated in the same way will last a certain number of hours before failure. ISO 281:2007 entitled, “Rolling Bearings—Dynamic Load Ratings and Life” describes such a model. There are two problems with this type of model. The first is that the model cannot predict which particular bearings will be the bearings that last the predicted number of hours and which will fail prior to the predicted life. The second is that the presumed actual viscosity used in the viscosity ratio may not be representative of the actual viscosity if the viscosity were measured.
If rather than presuming the actual viscosity it can be monitored, then rather than basing a bearing life calculation on a presumption, the bearing life calculation can be based on a monitored estimated actual viscosity ratio in the bearing concerned. This would be of particular value where bearings are of high value and are difficult to access, as servicing or repair of the bearing can be scheduled prior to failure.
Whilst WO2010/085971 attempts to provide a solution to the above-mentioned problem, it may be difficult to identify acoustic emission signals associated with asperity contacts occurring as the viscosity ratio falls only marginally below 0.5.
Also, monitoring a raw acoustic emission signal requires sampling and processing equipment capable of sampling and processing MHz signals. Such equipment is sophisticated, expensive and generally for use in the laboratory rather than in the field.
It would therefore be desirable to be able to ascertain lubricant viscosity, and hence lubricant viscosity ratio more accurately.
It would also be desirable to be able to ascertain lubricant viscosity and lubricant viscosity ratio using less sophisticated equipment.